Optical communication systems are widely used today for data communication. Such systems typically use optical fibers as the transmission medium to allow high data rates and long distance transmissions. To avoid wavelength blocking and to increase dense wavelength-division multiplexing fill, wavelength conversion can be required at optical cross-connect (OXC) sites.
In such optical communication systems, the use of pilot tones for identifying the wavelength of an optical data signal is known; the pilot tone will generally be a low frequency modulation of the optical data signal. When wavelength conversion of an optical data signal at a first wavelength to an optical data signal at another wavelength is required, the pilot tone also needs to be converted to properly identify this other wavelength. In several wavelength conversion approaches, the pilot tone signal is separated from the optical data signal, which is converted into an electrical signal that is used to modulate a laser emitting light at another wavelength. The conversion of the optical data signal involves extracting the actual data from the optical data signal. The data extraction method will depend on the format of the data (e.g., amplitude-shift keying, phase-shift keying, quadrature amplitude modulation, etc.) and can require extensive processing. A pilot tone of the optical data signal at this other wavelength is generated by applying a wavelength specific modulation to the optical data signal. Additionally, pilot tones can be modulated themselves, e.g., they can be turned on and off at a low rate, for example, 100 bits/s, to carry additional information such as, the modulation format of the optical data signal, the origin of the optical data signal, the optical spectrum width, etc.
Recently, in U.S. patent application Ser. No. 14/270,714, a coherent waveform conversion apparatus that can be used in coherent optical-fiber transmission system has been disclosed. In this system, wavelength conversion is effected on the entire waveform instead of only on the optical data signal. As such, wavelength conversion can be effected without consideration for the format type of the data encoded in the optical signal, which means that there is considerably less processing required. However, when a pilot tone identifying the wavelength of the optical data signal is present at the input of the coherent waveform conversion system, the pilot tone at the output of the coherent waveform conversion system will not properly identify the converted wavelength; rather, it will still identify the wavelength as that of the optical data signal at the input of the coherent waveform conversion system.
Improvements in coherent waveform conversion systems are therefore desirable.